Rich API for manipulating app files

[kilomeow]

Summary

Problem

There is no simple API methods for writing files to created app.

Impact

Until it's done, the developers will encounter this over and over again

Solution

Include publishAppFiles as an API method.
By the way this function relies on getPublicPathCid function and it's adoption is already discussed #179

Details

I dont know how advanced is fs.write, but if it is low-level it would be nice to rewrite only those files that need to be updated.